Work machines are used in earth moving operations to move material, such as dirt and rock, from one point to another. Conventional dump type work machines dump material by elevating a receptacle of the work machine with hydraulic cylinders. The force of gravity causes the material to be dumped out of the rear of the receptacle. There are several disadvantages to this type of work machine. First, elevating the receptacle of the work machine raises the center of gravity of the work machine making it less stable. Second, the raised receptacle can easily come into contact with overhead obstacles such as power lines or trees, which can limit the maneuverability of the work machine. Third, once the material begins to move in the receptacle, the material exits the receptacle quite rapidly, making it difficult for an operator of the work machine to spread the material evenly over a large area. In fact, a bulldozer is often employed to evenly distribute the material after it has been dumped from the work machine. Fourth, material, particularly dirt containing clay, can often stick to the receptacle of the work machine as the receptacle is elevated. The material stuck to the receptacle decreases the amount of material that can be moved by the work machine. In addition, an excavator or backhoe must be used to scrape the material from the receptacle, which can potentially damage the receptacle.
An ejector type work machine is an alternative to the conventional dump type work machine. Ejector type work machines do not elevate the receptacle to unload material. Instead, a hydraulic ejector cylinder, or cylinders, urges an ejector plate between the front and the rear of the receptacle. As the ejector plate moves from the front to the rear of the receptacle, material is ejected out of the receptacle at a controlled rate. Because the receptacle is not raised, the center of gravity is not increased as the material is ejected. Therefore, the stability of the ejector type work machine is not decreased as the material is ejected. In addition, the receptacle is not raised so as to be exposed to overhead obstacles such as trees and power lines. Moreover, the rate that the material is ejected from the rear of the ejector type work machine can be controlled, making it possible to evenly distribute the material over a large area thereby reducing the need to employ a bulldozer to spread the material. Also, because the ejector plate tightly matches the shape of the receptacle, any material that sticks to the receptacle is scraped out of the receptacle during the ejection process thereby increasing the amount of dirt that can subsequently be loaded into the ejector type work machine.
The receptacle of the ejector type work machine includes a tailgate which prevents material from exiting the receptacle prior to the ejector plate pushing the material out of the rear of the receptacle. The tailgate is moved between a closed position which prevents material from exiting the receptacle and an open position which allows material to exit the receptacle by a hydraulic tailgate cylinder.
A drawback to ejector type work machines that have heretofore been designed is that the control of the tailgate of the hydraulic ejector cylinder and the hydraulic tailgate cylinder was not well coordinated. Some previous hydraulic systems simply applied pressure simultaneously to the ejector cylinder and tailgate cylinder thereby immediately placing the tailgate in the open position as the ejector cylinder began to move. It is desirable that the opening of the tailgate can be adjusted to begin when the ejector plate is in any position between the front and the rear of the receptacle.
Another drawback of previous hydraulic systems is that the ejector cylinder moves slowly when moving from the rear of the receptacle to the front of the receptacle. In particular, large multistage cylinders typically used as ejection cylinders contain a large amount of hydraulic fluid that take a long time to drain. Specifically, this large quantity of hydraulic fluid must be routed through a control valve which limits the flow of the hydraulic fluid, and causes the ejector cylinder to retract slowly.
What is needed therefore is a method and apparatus for controlling the ejector cylinder and tailgate cylinder of the ejector type work machine which overcomes one or more of the above-mentioned drawbacks.